Thread Series(Tech Tip #6)

What are the Pro's and Con's of selecting a 'coarse' vs 'fine' thread for a fastener?

The term “Thread Series” refers to the combinations of diameter and pitch in threads, that are applied to specific thread diameters. These combinations differ from each other by the number of threads per inch applied to a series of specific diameters. Over time, there have been as many as 11 standard thread series, but as a practical matter today, only 3- coarse (UNC), fine (UNF) and 8 thread (8-UN) are in common use.

The evolution of thread series owes much to thread cutting and forming technologies as they evolved over the last hundred or so years. As technologies improved, threads that were more accurate and that featured finer pitches could be produced more economically and in greater quantities. This continuing advancement in screw thread technology resulted in engineers specifying fasteners with more than just 2- coarse and fine- thread series. A number of constant pitch series evolved in response to more demanding engineering applications. The most common pitch series in use today is 8-UN.

ASM’s Engineering Desk is often asked for an opinion on the pros and cons of fine vs coarse threads in fasteners. Fine threads tap better into thin walled structures and harder materials. They also have larger tensile stress areas which in turn make them stronger in tension than coarse threads. Fine threads also possess larger minor diameters, which provide high shear strengths.

Coarse threads tap better into brittle materials, and are less likely to cross thread. Coarse threading lends itself to thicker coatings and platings before thread adjustments need to be made. In “rough and tumble” work environments, coarse threads are more tolerant of abuse and will assemble and disassemble more easily and quickly than fine threads.

Which is right for your project? Email us at engineering@accuratescrew.com and we’ll be happy to work with you on the design that best meets your application’s requirements.